Publications on the WWW

• by D.Guidetti, R.A.Laing, J.H.Croston, A.H.Bridle, and P.Parma
• accepted for publication in Monthy Notices of the Royal Astronomical Society
• We explore the magneto-ionic environment of the isolated radio galaxy B2 0755+37 using detailed imaging of the distributions of Faraday rotation and depolarization over the radio source from Very Large Array observations at 1385, 1465 and 4860MHz and new X-ray data from XMM-Newton. The Rotation Measure (RM) distribution is complex, with evidence for anisotropic fluctuations in two regions. The approaching lobe shows low and uniform RM in an unusual ‘stripe’ along an extension of the jet axis and a linear gradient transverse to this axis over its Northern half. The leading edge of the receding lobe shows arc-like RM structures with sign reversals. Elsewhere, the RM structures are reasonably isotropic. The RM power spectra are well described by cut-off power laws with slopes ranging from 2.1 to 3.2 in different sub-regions. The corresponding magnetic-field autocorrelation lengths, where well-determined, range from 0.25 to 1.4 kpc. There is little large-scale RM structure in the receding lobe, where the outer scale of the fluctuations is between 16 and 30 kpc. The variation of RM fluctuation amplitude across the source is inconsistent with that expected from a beta-model gas distribution fitted to our X-ray data, even allowing for cavities in the thermal gas associated with the radio lobes. It is likely that the fluctuations are mostly produced by compressed gas and field around the leading edges of the lobes. The X-ray count rate is too low to determine the density distribution around 0755+37 in sufficient detail to estimate the field strength accurately, but values of a few µG are plausible. We identify areas of high depolarization around the jets and inner lobes. These could be produced by dense gas immediately surrounding the radio emission containing a magnetic field which is tangled on small scales. We also identify four ways in which the well known depolarization (Faraday depth) asymmetry between jetted and counter-jetted lobes of extended radio sources can be modified by interactions with the surrounding medium.

• by A.H.Bridle and M.H.Cohen
• in "Relativistic Jets from Active Galactic Nuclei", eds. Markus Böttcher, Daniel E. Harris, and Henric Krawczynski, Wiley (2012)
• Reviews the observed radio properties of relativistic jets from AGN on parsec to many-kiloparsec scales, with application to deriving jet kinematics from the radio data.

• by K.Y.Lo and A.H.Bridle
• in The Astronomy Revolution: 400 Years of Exploring the Cosmos, eds. Donald G.York, Owen Gingerich, and Shuang-Nan Zhang, Taylor and Francis (2011)
• In this 400th year since the invention of the optical telescope, and celebrating the New Vision in Astronomy on this occasion, we will discuss how new radio astronomical techniques have led to important discoveries in Astronomy. We then describe how further technical developments are leading to new facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA), the largest ground-based astronomical facility now under construction, and to the next-generation radio astronomy facilities, collectively named the Square Kilometer Array (SKA) or the SKA Program.

• by R.A.Laing, D.Guidetti, A.H.Bridle, P.Parma, and M.Bondi
• Monthy Notices of the Royal Astronomical Society, 417, 2789-2808 (2011)
• We present deep, high-resolution imaging of the nearby Fanaroff-Riley Class I FR I radio galaxies NGC 193, B2 0206+35, B2 0755+37 and M 84 at frequencies of 4.9 and 1.4 GHz using new and archival multi-configuration observations from the Very Large Array. In addition, we describe lower-resolution observations of B2 0326+39 and a reanalysis of our published images of 3C 296. All of these radio galaxies show twin jets and well-defined lobes or bridges of emission, and we examine the common properties of this class of source. We show detailed images of total intensity, brightness gradient, spectral index, degree of polarization and projected magnetic-field direction. The jet bases are very similar to those in tailed twin-jet sources and show the characteristics of decelerating, relativistic flows. Except on one side of M 84, we find that the jets can be traced at least as far as the ends of the lobes, where they often form structures which we call "caps'' with sharp outer brightness gradients. Continuing, but less well collimated flows back into the lobes from the caps can often be identified by their relatively flat spectral indices. The lobes in these radio galaxies are similar in morphology, spectral-index distribution and magnetic-field structure to those in more powerful (FR II) sources, but lack hot-spots or other evidence for shocks at the ends of the jets. M 84 may be an intermediate case between lobed and tailed sources, in which one jet does not reach the end of its lobe, but disrupts to form a "bubble''.

• by D.Guidetti, R.A.Laing, A.H.Bridle, P.Parma, and L.Gregorini
• Monthy Notices of the Royal Astronomical Society, 413, 2525-2544 (2011)
• We present detailed imaging of Faraday rotation and depolarization for the radio galaxies 0206+35, 3C 270, 3C 353 and M 84, based on Very Large Array observations at multiple frequencies in the range 1365 to 8440 MHz. This work suggests a more complex picture of the magneto-ionic environments of radio galaxies than was apparent from earlier work. All of the sources show spectacular banded rotation measure (RM) structures with contours of constant RM perpendicular to the major axes of their radio lobes. We give a comprehensive description of the banded RM phenomenon and present an initial attempt to interpret it as a consequence of interactions between the sources and their surroundings. We show that the material responsible for the Faraday rotation is in front of the radio emission and that the bands are likely to be caused by magnetized plasma which has been compressed by the expanding radio lobes. A two-dimensional magnetic structure in which the field lines are a family of ellipses draped around the leading edge of the lobe can produce RM bands in the correct orientation for any source orientation. We also report the first detections of rims of high depolarization at the edges of the inner radio lobes of M 84 and 3C 270. These are spatially coincident with shells of enhanced X-ray surface brightness, in which both the field strength and the thermal gas density are likely to be increased by compression.

• by R.A.Laing, A.H.Bridle, P.Parma, and M.Murgia
• Monthy Notices of the Royal Astronomical Society, 391, 521-549 (2008)
• We use high-quality VLA images of the Fanaroff & Riley Class I radio galaxy 3C 31 at six frequencies in the range 1365 to 8440 MHz to explore the spatial scale and origin of the rotation measure (RM) fluctuations on the line of sight to the radio source. We analyse the distribution of the degree of polarization to show that the large depolarization asymmetry between the North and South sides of the source seen in earlier work largely disappears as the resolution is increased. We show that the depolarization seen at low resolution results primarily from unresolved gradients in a Faraday screen in front of the synchrotron-emitting plasma. We establish that the residual degree of polarization in the short-wavelength limit should follow a Burn law and we fit such a law to our data to estimate the residual depolarization at high resolution. We show that the observed RM variations over selected areas of 3C 31 are consistent with a power spectrum of magnetic fluctuations in front of 3C 31 whose power-law slope changes significantly on the scales sampled by our data. The power spectrum can only have the form expected for Kolmogorov turbulence on scales <5 kpc. On larger scales we find a flatter slope. We also compare the global variations of RM across 3C 31 with the results of three-dimensional simulations of the magnetic-field fluctuations in the surrounding magnetoionic medium. We show that our data are consistent with a field distribution that favours the plane perpendicular to the jet axis - probably because the radio source has evacuated a large cavity in the surrounding medium. We also apply our analysis techniques to the case of Hydra A, where the shape and the size of the cavities produced by the source in the surrounding medium are known from X-ray data.

• edited by Alan H. Bridle, James J. Condon, and Gareth C. Hunt
• Astronomical Society of the Pacific Conference Series, Vol. 395 (2008)
• In celebration of its 50th anniversary, the NRAO hosted a science symposium highlighting the most important questions that can be addressed by astronomers: the creation of the universe, the formation of the first stars and galaxies, to the production of the elements, the assembly of molecules and the building blocks of life in interstellar space, how and where planets form, and possible changes in fundamental physical constants on cosmological time scales. This symposium brought together leading scientists to address the most important questions in these areas, the major contributions to astronomy enabled by NRAO instruments -- past, present, and future -- and to suggest future directions for research involving radio observations and techniques.

• by R.A.Laing, A.H.Bridle, P.Parma, L.Feretti, G.Giovannini, M.Murgia, and R.A.Perley
• Monthly Notices of the Royal Astronomical Society, 386, 657-672 (2008)
• Spectral and magnetic structure within the jets of 3C 31
• Brightest X-ray emission not associated with flattest radio spectra, but with particle acceleration mechanism of characteristic energy index 2.24. Flattest radio spectra associated with jet deceleration and regions of transverse velocity shear.
• Magnetic field aligned with emission arcs whose properties differ between jet and counterjet, consistent with differential relativistic aberration.

• by R.A.Laing and A.H.Bridle
• in "Extragalactic Jets: Theory and Observation from Radio to Gamma Ray", eds. T.A.Rector and D.S.De Young, ASP Conference Series, 386, 104-109 (2008)
• Analysis of the magnetic-field fluctuations in front of the radio galaxy 3C31.
• Fluctuations must be due primarily to foreground medium.
• Determine RM structure function and compare with 3-d simulations of RM produced by tangled field in plasma surrounding 3C31.
• Consistent with spherical plasma distribution in which radio source has produced a cavity.

• by R.A.Laing, A.H.Bridle, W.D.Cotton, D.M.Worrall, and M.Birkinshaw
• in "Extragalactic Jets: Theory and Observation from Radio to Gamma Ray", eds. T.A.Rector and D.S.De Young, ASP Conference Series, 386, 100-114 (2008)
• Accurate imaging of radio spectral index for the inner jets in three FR I radio galaxies.
• Where jets first brighten, small dispersion around spectral index of 0.62 in region where bright X-ray emission is detected.
• Further from nucleus, spectral index flattens to 0.50 - 0.55 and X-ray emission fainter relative to radio.
• Brightest X-ray emission not associated with flattest radio spectra, but with particle-acceleration process whose characteristic energy index is 2.24.
• Change in spectral index occurs roughly where relativistic jet models require rapid deceleration.

• by R.A.Laing and A.H.Bridle
• in "Extragalactic Jets: Theory and Observation from Radio to Gamma Ray", eds. T.A.Rector and D.S.De Young, ASP Conference Series, 386, 70-79 (2008)
• Unequivocal evidence that the jets in FR I radio galaxies are initially relativistic, decelerating flows
• Mass entrainment -from stellar mass loss within the jet volume or via a boundary layer at the jet surface is the most likely cause for deceleration.
• Consistent with velocity field and geometry inferred from kinematic modelling and external gas density and pressure profiles derived from X-ray data. .
• Jets must initially be very light, perhaps with an electron-positron composition.

• by D.M.Worrall, M.Birkinshaw, R.A.Laing, W.D.Cotton, and A.H.Bridle
• Monthly Notices of the Royal Astronomical Society, 380, 2-14 (2007)
• Deep imaging of the jet, nucleus and gaseous atmosphere of NGC315 with the Chandra X-ray observatory are compared with high-resolution VLA imaging at 5 GHz.
• Diffuse X-ray synchrotron emission is detected from the jet through regions both of fast bulk flow and deceleration. diffuse emission.
• The X-ray to radio ratio provides evidence for distributed particle acceleration throughout the fast moving region. .
• A remarkable knotty filament within the jet is seen in both the radio and the X-ray; its oscillatory apearance, roughly aligned magnetic field and requirements for particle acceleration suggest that it is a magnetic strand within a shear layer between fast inner and slower outer flow.

• by R.A.Laing, A.H.Bridle, and J.R.Canvin
• in "Relativistic Astrophysics and Cosmology: Einstein's Legacy", eds. B. Aschenbach, V. Burwitz, G. Hasinger, and B. Leibundgut, Springer-Verlag, 445-448 (2007)
• The largest-scale manifestation of Special Relativistic aberration in contemporary astrophysics is the appearance of initial asymmetries in kpc-scale radio-galaxy jets. We discuss how the Doppler beaming and Special Relativistic aberration modify the sidedness and polarization properties of intrinsically symmetrical relativistic flows and how these effects can be used to estimate the physical parameters of the jets in low-luminosity extragalactic radio sources.

• by R.A.Laing, J.R.Canvin, A.H.Bridle, and M.J.Hardcastle
• Monthly Notices of the Royal Astronomical Society, 372, 510-536 (2006)
• Deep imaging with the VLA is used to determine the distributions of spectral index, Faraday rotation measure, and apparent magnetic field over this nearby, low-luminosity radio galaxy.
• The flatter-spectrum radio jets are surrounded by a sheath of steeper-spectrum diffuse emission.
• We apply an intrinsically-symmetrical decelerating relativistic jet model to determine the velocity field of the jets. On-axis, they decelerate from v/c ∼ 0.8 to v/c ∼ 0.4 around 5 kpc from the nucleus with lower velocities everywhere at their edges.
• Our observations rule out a globally-ordered, helical magnetic field configuration but are well modeled by a field that is random on small scales but anisotropic, with toroidal and longitudinal components only.
• We interpret systematic morphological differences between local brightness enhancements ("arcs") in the main and counter-jets as a new effect of relativistic aberration.

• by R.A.Laing, J.R.Canvin, and A.H.Bridle
• in "The Origin and Evolution of Cosmic Magnetism", eds. R.Beck, G.Brunetti, L.Feretti, and B.Gaensler, Astronomisches Nachtrichten. 327, 523- 526 (2006)
• Large-scale helical fields are incompatible with our VLA polarimetry of the jets in FRI radio galaxies
• The combination of an ordered toroidal and disordered poloidal component is consistent with our data, as is a fully disordered, (but anisotropic) field.
• Jets must also contain small but significant amounts of radial field.

• by R.A.Laing, J.R.Canvin, W.D.Cotton, A.H.Bridle, and P.Parma
• in "The Origin and Evolution of Cosmic Magnetism", eds. R.Beck, G.Brunetti, L.Feretti, and B.Gaensler, Astronomisches Nachtrichten, 327, 533-534 (2006)
• The fluctuations of the rotation measure are larger in the fainter (receding) jets of two well-resolved FRI radio galaxies, as expected if the rotation occurs in the hot galaxy/group halos and the jet asymmetry is due to relativistic beaming.

• by R.A.Laing, J.R.Canvin, W.D.Cotton, and A.H.Bridle
• Monthly Notices of the Royal Astronomical Society, 368, 48-64 (2006).
• Five-frequency imaging with the VLA is used to determine the distributions of spectral index and Faraday rotation measure over the inner ± 400 arcsec of this giant low-luminosity radio galaxy
• The spectral structure suggests a change of dominant particle acceleration mechanism with distance from the nucleus. A transverse gradient from 0.55 on axis to 0.44 at the edge of the jet may be associated with a shear in the jet velocity field.
• The distribution of the Faraday rotation has three components. A constant term and a linear gradient are probably due to our Galaxy. Residual fluctuations are smaller in the brighter (approaching) jet, consistent with the idea that they are produced by magnetic fields in a halo of hot plasma that
• We also image the apparent magnetic field structure of the jets over the first ± 200 arsec from the nucleus.

• by J.R.Canvin, R.A.Laing, A.H.Bridle, and W.D.Cotton
• Monthly Notices of the Royal Astronomical Society, 363, 1223-1240 (2005).
• Applies an intrinsically symmetrical decelerating relativistic jet model to deep VLA imaging of the flaring region of the jets in the giant low-luminosity radio galaxy NGC 315.
• The inferred velocity, emissivity and field structure are very similar to those of other low-luminosity sources but all of the physical scales are larger by a factor of about 5.
• Simple adiabatic models fail to fit the emissivity variations.

• by M.J.Hardcastle, D.M.Worrall, M.Birkinshaw, R.A.Laing, and A.H.Bridle
• Monthly Notices of the Royal Astronomical Society, 358, 843-850 (2005).
• Observations of the twin-jet radio galaxy 3C296 with Chandra detect X-ray emission from the nucleus, from the inner parts of the radio jet, and from a small-scale thermal environment around the jet deceleration region.
• The X-ray jet and a steep pressure gradient in the external thermal environment are associated with the region where strong bulk deceleration of the jet material is suggested by radio observations.

• by R.A.Laing, J.R.Canvin, and A.H.Bridle
• "X-ray and Radio Connections", eds. Sjouwerman, L.O. and Dyer, K.K. (2004)
• Summarizes a project to determine physical conditions in the jets of low-luminosity (FR I) radio galaxies by modeling them as intrinsically symmetrical, relativistic, decelerating flows.
• Derives the three-dimensional distributions of velocity, rest-frame emissivity and magnetic-field structure in the jets. Outlines implications for theories of jet deceleration, including the use of conservation-law analysis to obtain profiles of internal pressure, density, Mach number and entrainment rate along the jets

• by R.A.Laing and A.H.Bridle
• Monthly Notices of the Royal Astronomical Society, 348, 1459-1472 (2004)
• General approach to modeling brightness and polarization structure of adiabatic, decelerating, relativistic jet
• Adiabatic models provide progressively better descriptions of 3C31 jets at larger distances from the galactic nucleus
• Evidence that particles are preferentially injected where bright X-ray emission found

• by D.E.Harris, A. Finoguenov, A.H.Bridle, M.J.Hardcastle, and R.A.Laing
• Astrophysical Journal, 580, 110-113 (2002)
• Chandra data show X-ray emission aligned with the northern radio jet
• Synchrotron emission is the favored explanation for the observed X-rays

• by R.A.Laing and A.H.Bridle
• Monthly Notices of the Royal Astronomical Society, 336, 1161-1180 (2002)
• Applies relativistic conservation laws to determine the energy, momentum and mass fluxes in the decelerating jets in an FRI radio galaxy
• Evidence that entrainment from galactic atmosphere is dominant mass input process far from galactic nucleus
• Stellar mass loss may also contribute to deceleration near flaring point of jet
• Compatible with initial electron-positron jet picking up thermal matter from stellar mass loss, or electron-proton jet with low-energy cutoff

• by M.J.Hardcastle, D.M.Worrall, M.Birkinshaw, R.A.Laing, and A.H.Bridle
• Monthly Notices of the Royal Astronomical Society, 334, 182-192 (2002)
• Deep Chandra imaging of the X-ray jet and hot gas in and around 3C31
• Derives density, temperature and pressure distribution in atmosphere of host galaxy
• Provides constraints on physical properties and stability of jets

• by R.A.Laing and A.H.Bridle
• Monthly Notices of the Royal Astronomical Society, 336, 328-352 (2002)
• Deep VLA imaging of the jets in 3C31 fitted by models of symmetric, antiparallel, axisymmetric, time-stationary relativistic flows
• Derives jet orientation, velocity field, emissivity variation and magnetic field configuration
• Evidence that jets decelerate by entraining the external medium
• Predicts appearance of jet at range of angles to line of sight, with implications for unified models

• by J.Dennett-Thorpe, P.A.G.Scheuer, R.A.Laing, A.H.Bridle, G.G.Pooley, and W.Reich
• Monthly Notices of the Royal Astronomical Society, 330, 609-620 (2002)
• Multi-frequency radio observations of two winged (X-shaped) radio sources
• Evidence for possible jet axis realignment

• by Christopher Palma, Franz Bauer, William Cotton, Alan Bridle, Steven Majewski, and Craig Sarazin
• Astronomical Journal, 119, 2068-2084 (2000)
• radio, optical and X-ray imaging and spectroscopy of a giant radio galaxy at z=0.145
• second largest known FR II radio source, 4 Mpc across
• discovered by the NRAO VLA Sky Survey

• by Jane Dennett-Thorpe, Alan Bridle, Robert Laing and Peter Scheuer
• Monthly Notices of the Royal Astronomical Society, 304, 271-280 (1999)
• correlations between spectral index, jet side and extent of the radio lobes in FR II sources
• hot spot spectra steepen as a function of radio power or redshift

• by Alan Bridle
• Barry Clark at 60, Socorro, June 1998
• imaging NGC315 as an example of the VLA's impact on jet physics

• by Mark Swain, Alan Bridle, and Stefi Baum
• Astrophysical Journal, 507, L29-L33 (1998)
• high-resolution VLA imaging of jet and counterjet in a nearby FR II radio galaxy
• modeling of emissivity and magnetic field distributions within the jet
• evidence that most of emission is from a thick boundary (shear) layer

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• by Jane Dennett-Thorpe, Alan Bridle, Peter Scheuer, Robert Laing, and Patrick Leahy
• untangling intrinsic and apparent asymmetries in synchrotron emission from powerful double radio sources
• Monthly Notices of the Royal Astronomical Society, 289, 753-765 (1997)

• a Workshop held Sept 19-23 1995 at the University of Alabama
• proceedings in A.S.P. Conf. Series, Vol.100 (1996)
• eds. Philip Hardee, Alan Bridle, Anton Zensus

• by Mark Swain, Alan Bridle, and Stefi Baum
• contributed paper in A.S.P. Conf. Series, Vol. 100, pp. 299-304 (1996)
• high-resolution VLA imaging of a nearby radio galaxy
• highest linear resolution ever on a large-scale FR II jet
• evidence for boundary (shear) layer and "sheath" emissions

• by Alan Bridle
• review paper in A.S.P. Conf. Series, vol. 100, pp. 383-394 (1996)
• discussion of jet velocities, velocity fields, and unification
• highlights of observational results presented at the meeting

• by Alan Bridle, David Hough, Colin Lonsdale, Jack Burns, and Robert Laing
• Astronomical Journal, 108, 766-820 (1994)
• sensitive high-resolution radio imaging and polarimetry
• implications for physics of energy transport by jets

• by David A. Clarke, Alan H. Bridle, Jack O. Burns, Richard A. Perley and Michael L. Norman
• Astrophysical Journal, 385, 173-187 (1992)
• High dynamic range VLA images at four frequencies with 1.4-arcsec resolution
• Extended filamentary emission of the lobes
• A second type of filamentation, spatially independent of the total intensity filaments, in the depolarization is associated with strong local gradients in the rotation measure
• Evidence for a clumpy magnetoionic medium around the radio galaxy
• The strengths and weaknesses of two interpretations of the abbreviated jets are considered

• by A.H.Bridle
• Astronomical Journal, 89, 979-986 (1984)
• Data on 125 extragalactic jets show that those in lower-power sources are generally two-sided and dominated by perpendicular magnetic field components over most of their lengths while those in higher-power sources are one-sided and dominated by parallel magnetic field over most of their lengths.
• The resolved jets in the weaker sources generally have faster lateral expansion (spreading) rates than those in the more powerful sources.

• by R.A.Perley, A.H.Bridle and A.G.Willis
• Astrophysical Journal Supplement Series, 54, 291-334 (1984)
• High dynamic range VLA images at four frequencies with 1.15 to 15-arcsec resolution
• Collimation and brightness evolution of the jet
• Magnetic field configuration and rotation measure distribution over the jet
• Implications for physics of the jet

• by A.H.Bridle, M.M.Davis, E.B.Fomalont and J.Lequeux
• Astronomical Journal, 77, 405 - 443 (1972)
• Accurate flux densities, precise positions of unresolved sources, and structures of resolved sources, for a 424 sources whose 1400-MHz integrated flux densities exceed 1.7 Jy.
• The 234 sources with 1400-MHz flux densities above 2 Jy, equivalent diameters less than 10 arcmin, and galactic latitude above 20 deg form a 98+/-2 per cent complete sample comparable in number to the Third Cambridge Catalogue in this 4.3-steradian area of sky.
• Later known as the "BDFL" source catalog.